Partial parting-off success

[paul]

I've struggled in the past to use a HSS parting-off tool; it's always ended up 'digging in', resulting in disaster.

I bought a blade type tool last week but when I came to use it found that although the shank is the same size as the regular HSS tools I'm using, the 'inclined' blade means that the cutting point is way above centre (and can't be lowered as no shim is required under the tool). Trying to cut with it in this manner is useless.

I therefore decided to revisit the HSS tool and was pleasantly surprised to find I could make a neat cut by running the lathe at virtually it's lowest speed and advancing the tool at a rate of about 1 thou per second when cutting a piece of BMS.

In part I thought that this was due to the workpiece being of rectangular section mounted in the 4 jaw i.e. with there being some 'relief' from the cut rather than continuous cutting as happens on round bar. Proceeding as above and withdrawing the tool regularly to allow swarf to drop away I managed a neat parting off operation.

Buoyed by this success I tried to part off a section of drilled and reamed round bronze bar (for use as a bush) but using the same technique as above found that it was like pushing the tool into a stodgy lump! It wouldn't cut as before, it just felt as though it was getting closer and closer to 'locking up'.

What might be wrong?

[Lurkio]

Paul,
Compared to cutting MS, the bronze is indeed a 'stodgy lump', which is why I feel you're not having the same success.
As you probably know, different materials need different cutting set-ups. I would cut the bronze much faster than MS, with lots of coolant, and with a steeper rake angle. There are books with all the different recommended settings in, though experience is often the main requirement.
Good to see you have as little tool overhang as possible in your photo. With your setup, there are a few other things you can try which may aid parting off. (Perhaps you are already aware of them).
To prevent digging in, everything must be really rigid, which is why cheap or badly worn machines present problems. Backlash in the slides is a problem - lock slides up where you can. Get your cross slide gibs set fairly tight so it doesn't get pulled forward as the tool tries to dig in.
Get the cut as close to the chuck as you can.
Ensure the parting tool isn't too wide, and support it underneath as far forward as you can.
Don't go too slow, unless it's a large diameter piece of work, and use plenty of coolant/lube.
Keep the tool sharp and smooth on top, or try a TCT if possible.

Hope this is useful,
Lurkio.

[abby]

Lurkio has said it all really, but I will add that for best results use a rear tool post if at all possible !
abby.

[Lurkio]

Thanks abby,

Just one other thought to add to abby's good suggestion...
I've had good success with a TC parting tool that's 'lost it's edge'. Because it's a bit dull it doesn't dig in, and because it's TC it can stand the heat generated without giving up the ghost.

Lurkio.

[SteveW]

Paul,

I've outlined it before and I'll do it again. Put a little hollow just behind the cutting edge of your HSS parting blade and you'll experience little to no trauma when parting off.

Prior to discovering the Greenwood parting tool combined with a home brewed rear tool post I was always in back-gear and been very tense. On discovering the form of the the Greenwood TCT tip and replicating it on the HSS parting blade I get nearly the same results and no longer use back-gear.

The only problem with the very excellent Greenwood tip is that it's a bit wide so you get to wast a lot of material.

[abby]

Found some thing really useful recently - small , diamond coated polishing laps . They are I believe made for use by stone masons but they are excellent for putting an edge back on to TCT or HSS - I obtained several from the 3M's factory shop at sample price so I don't know what they retail at. There are also larger hard foam blocks which are perfect for taking high spots off bedways , slides etc.

[Shawki Shlemon]

Hi abby
I have one of these handy tools , I use it to clean the cutting edge on drill bits ( take off the sharp edge ) for drilling gunmetal to prevent grabbing and it works fine .I have had it for years and I forgot where I got it from.

[dickdastardly40]

img179.imagevenue.com/img.php?image=09438_Tool_Tip_122_411lo.jpg
If you can grind a hollow like this as advised by SteveW the chip curls slightly and doesn't bind so much before it comes out the recess.

The pic was poached from another site and I believe it was ground with a die grinder/dremel

[SteveW]

Guys,

I just tried to photograph my parting tool but the result wasn't clear enough. The hollow I've got is different from the picture above.

The depression I achieved using a dremel/grinder is further back from the cutting edge. In effect the hollow provides the back rake but by also being oval the rake causes the swarf to fold and roll nicely before it exits the slot without touching the sides.

I use diamond laps (Chronos) to to give the thing the kiss of life every now and again.

I also find the tin of spray tapping fluid from SCREWFIX (UK) a great way of providing a controlled dose of lubricant for this type of heavy cutting. It works great on surfaces I fly-cut because it doesn't run off and on hacksawing bigger lumps of steel. However, if you get if too hot it smokes (like most lubs).

[chameleonrob]

one thing I would try is a thinner parting off tool to, say, 6mm in then widen the grove with the wider one then back to the thin one.

rob

[dscott]

The first thing I built for my lathe was a Gibralta tool post from Hemmingway the second was a rear tool post followed by a quick change tool holder set. All three make life with a lathe a pleasure, I even did a rear post for the unimat. This sits on its own platform with the toolpost in front.

56 X 5/16" spacers needed in stainless, so set the two apart the 5/16" and make as many as you like, dont forget to drill the holes in the middle.

Happy turning.

David.

[SteveW]

Rob,

At 6mm you can get through a lot of expensive metal. I don't use my Greenwood parting tool much for this reason, it's around 4mm wide.

I use a blade/holder type at around 3mm wide formed as described elsewhere. Your point about going for a wider slot is a good one but I do it simply by offsetting the one tool in the slot. You need to be careful because the thinner blades won't tolerate side loads but you only really need a few tens of thou extra width.

The other good tip I was given was to do final pass down the back of the material being cut off, a finishing cut. If you time it right the completed part can be given the cut using this final facing cut with a follow-through to centre line.

I can also recommend rear tool posts and they're a doddle to make once you've got a bit of 1-3/4" square steel long enough to get a tool mounted upside down at centre height and securely clamped.

[chameleonrob]

very true SteveW, as in most things the trick is making the right compromise. for small dia items then using a thin tool in steps is fine, I do it myself, but when parting off large diameter pieces having one think and one thin parting off tool means that the thin one only needs to be thin at the end, and that alignment of the tool isn't quite so critical. the method lends itself to machines with quick change tool posts and rear tool posts, one tools in each, great if you're doing production runs.

rob

[Lurkio]

SteveW,

Unless I'm reading it wrong, I think rob is suggesting a 'narrow' parting tool is used to a depth of 6mm, not a 6mm wide tool.

Lurkio.

[SteveW]

Rob,

Re the 6mm thing

Oops! silly me.